Cables used for electric power distribution are typically operated at thousands of volts, and consequently care must be taken when terminating these cables and connecting them to electrical components, e.g. switchgear units. At the point where the cable is terminated electrical stress occurs that needs to be controlled.
The cable termination typically requires that various dielectric, insulation, semi-conducting and conductive shield layers that surrounds the central current carrying conductor are removed, and that the various semi-conducting and conducting surrounding layers are appropriately connected to the involved electrical equipment. In particular, electric stress occurs at sudden changes in the cable shielding system, e.g. where a semi-conducting outer layer is cut away.
A cable termination for medium and high voltages is normally an insulator comprising a housing inside which is located a stress control device mounted on the cable. The insulator housing is usually of porcelain or a composite. The function of the stress control device is to control the electric field, e.g. where an outer semi-conducting layer is cut away. The stress control device is often cone-shaped, and referred to as a stress cone. The housing is filled with an electrical insulation medium. This insulation medium is traditionally oil, in which case the cable termination device is referred to as being of the wet type. The cable termination device may alternatively be of the so called dry type, for example when the insulation material is an elastomeric compound as in U.S. Pat. No. 5,130,495, or a gel of some suitable kind, e.g. a polymer or silicone gel.
Traditionally, the different parts of the cable termination device are assembled at the end site and mounted on the cable. The device is then raised in an upright position. Finally, the electrical insulating medium, such as oil, is filled into the insulator housing and the housing is then sealed by mounting of the top end cap and the external electrical connection.
The assembling work at site is a time consuming procedure with a medium such as oil that is unfriendly to the environment. There are risks involved, for example if the cable termination should explode the damage would be severe with burning oil.
The existing dry types of cable termination devices have the main drawback of being expensive.